Throughout the novel, Joe has had an obsession with Parry and in this chapter, that obsession is resolved. .
Other obsessions which have lingered throughout the book are also cured. Joe, in explaining the science of the river to the children, comes to terms with his work as a science journalist rather than a scientist and ceases to view himself as a failure. .
The most important obsession to be cured is the obsession with John Logan. Both Joe and Jean Logan are obsessive about John Logan. Jean Logan is obsessive with jealousy and Joe is obsessive with guilt. Throughout the novel Joe has wondered whether he was responsible for John Logan's death, and Jean Logan has wondered if John Logan was having an affair. Logan has haunted the entire novel in various guises, as the selfless martyr, the absent father and the possibly but eventually not faithless husband. .
Chapter twenty four concludes the story of Logan. It is explained that he was not unfaithful, so although plunged into guilt, "but who's going to forgive me? the only person who can is dead", Jean Logan can grieve properly, then move on. Joe is also absolved of his guilt about Logan's death in this chapter, because he has done something for Logan. He has proved Logan's fidelity to his wife. There are even parts of this chapter where it appears that Joe takes Logan's place in his interactions with the children. John Logan, being dead, is the only character in the novel who can have a real ending. The other characters can have issues resolved, but it is only a temporary, artificial ending. .
It is appropriate that the issues which are resolved in this chapter, are resolved at a picnic "much the same as before". The book began with a picnic, so it's ending with a picnic is another factor which adds to the cathartic atmosphere of the chapter. It is as if by revisiting the setting of the chaotic first chapter "The last time [Joe] understood anything clearly at all", Joe and the other characters can make sense of the confusion and guilt the balloon incident had plunged them into.